The present invention relates to the millimeter and sub millimeter wavelength generation, amplification, and processing arts. It particularly relates to electron devices such as traveling wave tubes for millimeter and sub mm wavelength amplifiers and oscillators, and will be described with particular reference thereto. However, the invention will also find application in other devices that operate at millimeter and sub mm wavelengths, and in other devices that employ slow wave circuits.
A traveling wave tube (TWT) is an electron device that typically includes a slow wave circuit defined by a generally hollow vacuum-tight barrel with optional additional millimeter and sub mm wavelength circuitry disposed inside the barrel. An electron source and suitable steering magnets or electric fields are arranged around the slow wave circuit to pass an electron beam through the generally hollow beam tunnel. The electrons interact with the slow wave circuit, and energy of the electron beam is transferred into microwaves that are guided by the slow wave circuit. Such traveling wave tubes provide millimeter and sub mm wavelength generation and amplification.
A generation ago the helical backward wave oscillator (BWO) was the signal source of choice for microwave swept frequency oscillators. However, today this application has been taken over by solid state devices. Helical slow wave circuits are still used as high power millimeter wave traveling wave tube (TWT) amplifiers, producing as much as 200 Watts CW at 45 GHz, but fundamental issues associated with conventional fabrication, thermal management and electron beam transmission are obstacles to higher frequency applications. For decades the conventional practice of helix fabrication has involved winding round wire or rectangular tape around a cylindrical mandrel. As the desired frequency of operation increases, the mandrel diameter must decrease, exaggerating the stress between the inner and outer radii of the helix as the wire thickness becomes a significant fraction of the mandrel radius. Heat generated on the helix whether by electron beam interception or ohmic losses from the RF current must be conducted away through dielectric support rods that are inferior thermal conductors and which frequently make somewhat uncertain thermal contact with the helix. The inside diameter of the helix is reduced as frequency increases, providing a reduced space for conventional electron beam transmission and, therefore, reducing the achievable output power.
The present invention contemplates a new and improved vacuum electron device that resolves the above-referenced difficulties and others.